bcachefs: bch2_fpunch_snapshot()

Add a new version of fpunch for operating on a snapshot ID, not a
subvolume - and use it for "extent past end of inode" repair.

Previously, repair would try to delete everything at once, but deleting
too many extents at once can overflow the btree_trans bump allocator, as
well as causing other problems - the new helper properly uses
bch2_extent_trim_atomic().

Reported-and-tested-by: Edoardo Codeglia <bcachefs@404.blue>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>