btree updates before going RW are expensive if they're in random order,
since they use the list of keys for journal replay to insert, which is
just a gap buffer.

This patch improves the bucket invalidate path so that if
bch2_check_lrus() hasn't finished it only prints warnings instead of
doing an emergency shutdown, which means we can now set BCH_FS_MAY_GO_RW
before bch2_check_lrus().

Also, the filesystem state bits are reorganized a bit.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>