bcachefs: fix up wonky error handling in bch2_seek_pagecache_hole()

bcachefs: fix up wonky error handling in bch2_seek_pagecache_hole()

The folio_hole_offset() helper returns a mix of bool and int types.
The latter is to support a possible -EAGAIN error code when using
nonblocking locks. This is not only confusing, but the only caller
also essentially ignores errors outside of stopping the range
iteration. This means an -EAGAIN error can't return directly from
folio_hole_offset() and may be lost via bch2_clamp_data_hole().

Fix up the error handling and make it more readable.
__filemap_get_folio() returns -ENOENT instead of NULL when no folio
exists, so reuse the same error code in folio_hole_offset(). Fix up
bch2_seek_pagecache_hole() to return the current offset on -ENOENT,
but otherwise return unexpected error code up to the caller.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>