fs: remove _submit_bh()

_submit_bh() allowed submitting a buffer_head for I/O using custom
bio_flags.  It used to be used by jbd to set BIO_SNAP_STABLE, introduced
by commit 71368511 ("mm: make snapshotting pages for stable writes a
per-bio operation").  However, the code and flag has since been removed
and no _submit_bh() users remain.

These days, bio_flags are mostly used internally by the block layer to
track the state of bio's.  As such, it doesn't really make sense for
filesystems to use them instead of op_flags when wanting special
behavior for block requests.

Therefore, remove _submit_bh() and trim the bio_flags argument from
submit_bh_wbc().

Cc: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>