fs: ext4: initialize fsdata in pagecache_write()

When aops->write_begin() does not initialize fsdata, KMSAN reports
an error passing the latter to aops->write_end().

Fix this by unconditionally initializing fsdata.

Cc: Eric Biggers <ebiggers@kernel.org>
Fixes: c93d8f88 ("ext4: add basic fs-verity support")
Reported-by:  <syzbot+9767be679ef5016b6082@syzkaller.appspotmail.com>
Signed-off-by: Alexander Potapenko <glider@google.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20221121112134.407362-1-glider@google.com


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