NFS: Add a small buffer in nfs_fs_context to avoid string dup

Add a small buffer in nfs_fs_context to avoid string duplication when
parsing numbers.  Also make the parsing function wrapper place the parsed
integer directly in the appropriate nfs_fs_context struct member.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>