ext4: fix transaction overflow during writeback

Commit 95ad8ee45cdb ("ext4: correct the reserved credits for extent
conversion") was correct to note that we need to reserve enough credits
for all extents possibly underlying a large folio. However it was too
eager to reduce the number of reserved credits. Extent conversion may
not only need to touch several leaf extent blocks, it may also need to
split extents - for example a single large unwritten extent may need to
be split into many small written ones in case of sparse folio dirtying.
This can thus result not only in extent leaf modifications but also in a
need to allocate new extent tree nodes. As a result the reserved
transaction credits were not sufficient in some corner cases. Use
ext4_meta_trans_blocks() for correct upper bound credit estimate.

Fixes: 95ad8ee45cdb ("ext4: correct the reserved credits for extent conversion")
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20260805153605.166545-5-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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