NFSv4.2: fix error return on memory allocation failure

Currently when an alloc_page fails the error return is not set in
variable err and a garbage initialized value is returned. Fix this
by setting err to -ENOMEM before taking the error return path.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: a1f26739 ("NFSv4.2: improve page handling for GETXATTR")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>