LU-11582 llite: protect reading inode->i_data.nrpages

LU-11582 llite: protect reading inode->i_data.nrpages

truncate_inode_pages() looks up pages in the radix tree without
lock, and could miss finding pages removed from the radix tree
by __remove_mapping(), so that after calling truncate_inode_pages()
we need to read the nrpages of the inode->i_data with the protection
of tree_lock.

Since it could still be in the race window of __remove_mapping()->
__delete_from_page_cache()->page_cache_tree_delte(), before the
nrpages being decreased.

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: I44ba6bea3dec4f0a110d1ae2a749514ec7dd0d12
Reviewed-on: https://review.whamcloud.com/33639
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Tested-by: Maloo <maloo@whamcloud.com>