[PATCH][SMB3] allow files to be created with backslash in file name

[Steve French]

Backslash is reserved in Windows (and SMB2/SMB3 by default) but
allowed in POSIX so must be remapped when POSIX extensions are
not enabled.

The default mapping for SMB3 mounts ("SFM") allows mapping backslash
(ie 0x5C in UTF8) to 0xF026 in UCS-2 (using the Unicode remapping
range reserved for these characters), but this was not mapped by
cifs.ko (unlike asterisk, greater than, question mark etc).  This patch
fixes that to allow creating files and directories with backslash
in the file or directory name.

Before this patch:
   touch "/mnt2/filewith\slash"
would return
   touch: setting times of '/mnt2/filewith\slash': Invalid argument

With the patch touch and mkdir with the backslash in the name works.

This problem was found while debugging xfstest 453 - see
    https://bugzilla.kernel.org/show_bug.cgi?id=210961

This patch may be even more important to Samba, as alternative ways of
storing these files can create more problems. Interestingly Samba
server reports local files with backslashes in them over the wire
without remapping, even though these are illegal in SMB3 which would
cause confusion on the client(s).  Has anyone tried Windows mounting
to Samba and viewing files with locally created Linux files that
include these reserved characters (presumably Windows clients won't
like it either)?

This patch does allow creating/viewing files with remotely with '\' in
the file name from Linux (cifs.ko) but does not completely fix xfstest
453 kernel (more investigation of this test is needed).  Test 453
creates filenames with 'horrifying'
(using their term) sequences of arbitrary bytes in file names.

Reported-by: Xiaoli Feng <xifeng at redhat.com>

-- 
Thanks,

Steve
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