ext4: recalculate journal credits as inode depth changes

Currently in ext4_alloc_file_blocks() the number of credits is
calculated only once before we enter the allocation loop. However within
the allocation loop the extent tree depth can change, hence the number
of credits needed can increase potentially exceeding the number of credits
reserved in the handle which can cause journal failures.

Fix this by recalculating number of credits when the inode depth
changes. Note that even though ext4_alloc_file_blocks() is only
currently used by extent base inodes we will avoid recalculating number
of credits unnecessarily in the case of indirect based inodes.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>