tmpfs: Use xattr handler infrastructure

tmpfs: Use xattr handler infrastructure

Use the VFS xattr handler infrastructure and get rid of similar code in
the filesystem.  For implementing shmem_xattr_handler_set, we need a
version of simple_xattr_set which removes the attribute when value is
NULL.  Use this to implement kernfs_iop_removexattr as well.

Signed-off-by: Andreas Gruenbacher <[email protected]>
Reviewed-by: James Morris <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: [email protected]
Signed-off-by: Al Viro <[email protected]>