NFSv4.2: fix error handling in nfs42_proc_getxattr

NFSv4.2: fix error handling in nfs42_proc_getxattr

There is a slight issue with error handling code inside
nfs42_proc_getxattr(). If page allocating loop fails then we free the
failing page array element which is NULL but __free_page() can't deal with
NULL args.

Found by Linux Verification Center (linuxtesting.org).

Fixes: a1f26739ccdc ("NFSv4.2: improve page handling for GETXATTR")
Signed-off-by: Fedor Pchelkin <[email protected]>
Reviewed-by: Benjamin Coddington <[email protected]>
Signed-off-by: Trond Myklebust <[email protected]>