From: Oliver Xymoron <oxymoron@waste.org>

This patch just saves a few bytes in the inode by turning mapping->gfp_mask
into an unsigned long mapping->flags.

The mapping's gfp mask is placed in the 16 high bits of mapping->flags and
two of the remaining 16 bits are used for tracking EIO and ENOSPC errors.

This leaves 14 bits in the mapping for future use.  They should be accessed
with the atomic bitops.