fs/ext4: use sleeping version of sb_find_get_block()

Enable ext4_free_blocks() to use it, which has a cond_resched to begin
with. Convert to the new nonatomic flavor to benefit from potential
performance benefits and adapt in the future vs migration such that
semantics are kept.

Suggested-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Link: https://kdevops.org/ext4/v6.15-rc2.html # [0]
Link: https://lore.kernel.org/all/aAAEvcrmREWa1SKF@bombadil.infradead.org/ # [1]
Link: https://lore.kernel.org/20250418015921.132400-7-dave@stgolabs.net


Tested-by:  <kdevops@lists.linux.dev>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>