shmem: make shmem_get_inode() return ERR_PTR instead of NULL

Make shmem_get_inode() return ERR_PTR instead of NULL on error. This will be
useful later when we introduce quota support.

There should be no functional change.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Message-Id: <20230725144510.253763-3-cem@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>