erofs: add erofs_sb_free() helper

erofs: add erofs_sb_free() helper

[ Upstream commit e2de3c1bf6a0c99b089bd706a62da8f988918858 ]

Unify the common parts of erofs_fc_free() and erofs_kill_sb() as
erofs_sb_free().

Thus, fput() in erofs_fc_get_tree() is no longer needed, too.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20241212133504.2047178-1-hsiangkao@linux.alibaba.com
Stable-dep-of: 6422cde1b0d5 ("erofs: use buffered I/O for file-backed mounts by default")
Signed-off-by: Sasha Levin <sashal@kernel.org>