nfs: check wait_on_bit_lock err in page_group_lock

Return errors from wait_on_bit_lock from nfs_page_group_lock.

Add a bool argument @wait to nfs_page_group_lock. If true, loop over
wait_on_bit_lock until it returns cleanly. If false, return the error
from wait_on_bit_lock.

Signed-off-by: Weston Andros Adamson <dros@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

diff --cc fs/nfs/pagelist.c
Simple merge

diff --cc fs/nfs/write.c
Simple merge

diff --cc include/linux/nfs_page.h
Simple merge