xfs: don't crash the vfs on a garbage inline symlink

The VFS routine that calls ->get_link blindly copies whatever's returned
into the user's buffer.  If we return a NULL pointer, the vfs will
crash on the null pointer.  Therefore, return -EFSCORRUPTED instead of
blowing up the kernel.

[dgc: clean up with hch's suggestions]

Reported-by:  <wen.xu@gatech.edu>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>