cifs: Fix check for regular file in couldbe_mf_symlink()

cifs: Fix check for regular file in couldbe_mf_symlink()

MF Symlinks are regular files containing content in a specified format.

The function couldbe_mf_symlink() checks the mode for a set S_IFREG bit
as a test to confirm that it is a regular file. This bit is also set for
other filetypes and simply checking for this bit being set may return
false positives.

We ensure that we are actually checking for a regular file by using the
S_ISREG macro to test instead.

Signed-off-by: Sachin Prabhu <[email protected]>
Reviewed-by: Jeff Layton <[email protected]>
Reported-by: Neil Brown <[email protected]>
Reported-by: Dan Carpenter <[email protected]>
Signed-off-by: Steve French <[email protected]>