efivarfs: Replace invalid slashes with exclamation marks in dentries.

Without this patch efivarfs_alloc_dentry creates dentries with slashes in
their name if the respective EFI variable has slashes in its name. This in
turn causes EIO on getdents64, which prevents a complete directory listing
of /sys/firmware/efi/efivars/.

This patch replaces the invalid shlashes with exclamation marks like
kobject_set_name_vargs does for /sys/firmware/efi/vars/ to have consistently
named dentries under /sys/firmware/efi/vars/ and /sys/firmware/efi/efivars/.
Signed-off-by: Michael Schaller <misch@google.com>
Link: https://lore.kernel.org/r/20200925074502.150448-1-misch@google.comSigned-off-by: Ard Biesheuvel <ardb@kernel.org>

fs/efivarfs/super.c

@@ -141,6 +141,9 @@ static int efivarfs_callback(efi_char16_t *name16, efi_guid_t vendor,
 
 	name[len + EFI_VARIABLE_GUID_LEN+1] = '\0';
 
+	/* replace invalid slashes like kobject_set_name_vargs does for /sys/firmware/efi/vars. */
+	strreplace(name, '/', '!');
+
 	inode = efivarfs_get_inode(sb, d_inode(root), S_IFREG | 0644, 0,
 				   is_removable);
 	if (!inode)