cachefiles: Fix non-taking of sb_writers around set/removexattr

Unlike other vfs_xxxx() calls, vfs_setxattr() and vfs_removexattr() don't
take the sb_writers lock, so the caller should do it for them.

Fix cachefiles to do this.

Fixes: 9ae326a6 ("CacheFiles: A cache that backs onto a mounted filesystem")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Christian Brauner <brauner@kernel.org>
cc: Gao Xiang <xiang@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-erofs@lists.ozlabs.org
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20240814203850.2240469-3-dhowells@redhat.com/

 # v2
Signed-off-by: Christian Brauner <brauner@kernel.org>