slab: distinguish lock and trylock for sheaf_flush_main()

sheaf_flush_main() can be called from __pcs_replace_full_main() where
the trylock can in theory fail, and pcs_flush_all() where it's not
expected to and it would be actually a problem if it failed and left the
main sheaf not flushed.

To make this explicit, split the function into sheaf_flush_main() (using
local_lock()) and sheaf_try_flush_main() (using local_trylock()) where
both call __sheaf_flush_main_batch() to flush a single batch of objects.
This will allow lockdep to verify our assumptions.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>