bcachefs: Fallocate now checks page cache

bcachefs: Fallocate now checks page cache

Previously, fallocate would only check the state of the extents btree
when determining if we need to create a reservation.

But the page cache might already have dirty data or a disk reservation.
This changes __bchfs_fallocate() to call bch2_seek_pagecache_hole() to
check for this.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>