iomap: use find_next_bit() for dirty bitmap scanning

iomap: use find_next_bit() for dirty bitmap scanning

Use find_next_bit()/find_next_zero_bit() for iomap dirty bitmap
scanning. This uses __ffs() internally and is more efficient for
finding the next dirty or clean bit than iterating through the bitmap
range testing every bit.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://patch.msgid.link/20251111193658.3495942-9-joannelkoong@gmail.com
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>