ext4: tidy the BH loop in mext_page_mkuptodate()

This for loop is somewhat hard to read; turn it into a normal BH
do-while loop.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://patch.msgid.link/20240718223005.568869-4-willy@infradead.org


Signed-off-by: Theodore Ts'o <tytso@mit.edu>