cifs: mapchars mount option ignored

There are two ways that special characters (not allowed in some
other operating systems like Windows, but allowed in POSIX) have
been mapped in the past ("SFU" and "SFM" mappings) to allow them
to be stored in a range reserved for special chars. The default
for Linux has been to use "mapposix" (ie the SFM mapping) but
the conversion to the new mount API in the 5.11 kernel broke
the ability to override the default mapping of the reserved
characters (like '?' and '*' and '\') via "mapchars" mount option.

This patch fixes that - so can now mount with "mapchars"
mount option to override the default ("mapposix" ie SFM) mapping.

Reported-by: Tyler Spivey <tspivey8@gmail.com>
Fixes: 24e0a1ef

 ("cifs: switch to new mount api")
Signed-off-by: Steve French <stfrench@microsoft.com>