fat: Propagate inode buffer write errors from fat_sync_inode_metadata()

fat: Propagate inode buffer write errors from fat_sync_inode_metadata()

fat_sync_inode_metadata() ignores the result of writing the buffer
containing the inode's directory entry. Before commit 525da4f40a7c
("fat: Fix missed inode writeback during fsync(2)") a write error was
propagated to fsync(2) via __fat_write_inode() -> sync_dirty_buffer(),
now fsync(2) reports success even though the inode's directory entry
could not be written. Check buffer_write_io_error() after
sync_dirty_buffer() like the other ->sync_inode_metadata
implementations do.

Fixes: 525da4f40a7c ("fat: Fix missed inode writeback during fsync(2)")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>