Reading EEPROM fails with following warning:

[   16.357496] ------------[ cut here ]------------
[   16.357529] fsl_spi b01004c0.spi: rejecting DMA map of vmalloc memory
[   16.357698] WARNING: CPU: 0 PID: 371 at include/linux/dma-mapping.h:326 fsl_spi_cpm_bufs+0x2a0/0x2d8
[   16.357775] CPU: 0 PID: 371 Comm: od Not tainted 5.16.11-s3k-dev-01743-g19beecbfe9d6-dirty #109
[   16.357806] NIP:  c03fbc9c LR: c03fbc9c CTR: 00000000
[   16.357825] REGS: e68d9b20 TRAP: 0700   Not tainted  (5.16.11-s3k-dev-01743-g19beecbfe9d6-dirty)
[   16.357849] MSR:  00029032 <EE,ME,IR,DR,RI>  CR: 24002282  XER: 00000000
[   16.357931]
[   16.357931] GPR00: c03fbc9c e68d9be0 c26d06a0 00000039 00000001 c0d36364 c0e96428 00000027
[   16.357931] GPR08: 00000001 00000000 00000023 3fffc000 24002282 100d3dd6 100a2ffc 00000000
[   16.357931] GPR16: 100cd280 100b0000 00000000 aff54f7e 100d0000 100d0000 00000001 100cf328
[   16.357931] GPR24: 100cf328 00000000 00000003 e68d9e30 c156b410 e67ab4c0 e68d9d38 c24ab278
[   16.358253] NIP [c03fbc9c] fsl_spi_cpm_bufs+0x2a0/0x2d8
[   16.358292] LR [c03fbc9c] fsl_spi_cpm_bufs+0x2a0/0x2d8
[   16.358325] Call Trace:
[   16.358336] [e68d9be0] [c03fbc9c] fsl_spi_cpm_bufs+0x2a0/0x2d8 (unreliable)
[   16.358388] [e68d9c00] [c03fcb44] fsl_spi_bufs.isra.0+0x94/0x1a0
[   16.358436] [e68d9c20] [c03fd970] fsl_spi_do_one_msg+0x254/0x3dc
[   16.358483] [e68d9cb0] [c03f7e50] __spi_pump_messages+0x274/0x8a4
[   16.358529] [e68d9ce0] [c03f9d30] __spi_sync+0x344/0x378
[   16.358573] [e68d9d20] [c03fb52c] spi_sync+0x34/0x60
[   16.358616] [e68d9d30] [c03b4dec] at25_ee_read+0x138/0x1a8
[   16.358667] [e68d9e50] [c04a8fb8] bin_attr_nvmem_read+0x98/0x110
[   16.358725] [e68d9e60] [c0204b14] kernfs_fop_read_iter+0xc0/0x1fc
[   16.358774] [e68d9e80] [c0168660] vfs_read+0x284/0x410
[   16.358821] [e68d9f00] [c016925c] ksys_read+0x6c/0x11c
[   16.358863] [e68d9f30] [c00160e0] ret_from_syscall+0x0/0x28
...
[   16.359608] ---[ end trace a4ce3e34afef0cb5 ]---
[   16.359638] fsl_spi b01004c0.spi: unable to map tx dma

This is due to the AT25 driver using buffers on stack, which is not
possible with CONFIG_VMAP_STACK.

As mentionned in kernel Documentation (Documentation/spi/spi-summary.rst):

  - Follow standard kernel rules, and provide DMA-safe buffers in
    your messages.  That way controller drivers using DMA aren't forced
    to make extra copies unless the hardware requires it (e.g. working
    around hardware errata that force the use of bounce buffering).

Modify the driver to use a buffer located in the at25 device structure
which is allocated via kmalloc during probe.

Protect writes in this new buffer with the driver's mutex.

Fixes: b587b13a ("[PATCH] SPI eeprom driver")
Cc: stable <stable@kernel.org>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/r/230a9486fc68ea0182df46255e42a51099403642.1648032613.git.christophe.leroy@csgroup.euSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>