Fix zfs_putpage() lock inversion

There exists a lock inversions involving the zfs range lock and the
individual page writeback bits which can result in a deadlock.  To
prevent this we must always manipulate the writeback bit while
holding the range lock.  The exact deadlock is as follows:

------ Process A ------        ------ Process B ------
zpl_writepages                 zpl_fallocate
write_cache_pages              zpl_fallocate_common
zpl_putpage                    zfs_space
zfs_putpage (set bit)          zfs_freesp
zfs_range_lock (wait on lock)  zfs_free_range (take lock)
                               truncate_inode_pages_range
                               wait_on_page_writeback (wait on bit)

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2976