fs: f2fs: initialize fsdata in pagecache_write()

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

Fix this by unconditionally initializing fsdata.

Suggested-by: Eric Biggers <ebiggers@kernel.org>
Fixes: 95ae251f

 ("f2fs: add fs-verity support")
Signed-off-by: Alexander Potapenko <glider@google.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>