pNFS: rework pnfs_generic_pg_check_layout to check IO range

All callers of pnfs_generic_pg_check_layout() also want to do a call to
check that the layout's range covers the IO range. Merge the functionality
of the pnfs_generic_pg_check_range() into that of
pnfs_generic_pg_check_layout().

Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>