fs/buffer: use sleeping version of __find_get_block()

Convert to the new nonatomic flavor to benefit from potential performance
benefits and adapt in the future vs migration such that semantics
are kept.

Convert write_boundary_block() which already takes the buffer
lock as well as bdev_getblk() depending on the respective gpf flags.
There are no changes in semantics.

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/20250418015921.132400-4-dave@stgolabs.net


Tested-by: kdevops@lists.linux.dev # [0] [1]
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>