fs: move {lock, unlock}_mount_hash to fs/mount.h

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8PWEP


CVE: NA

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commit d033cb67 ("mount: make {lock,unlock}_mount_hash() static")
moves {lock, unlock}_mount_hash to fs/namespace.c due to the two
functions are only used in the file.

memcg_memfs_info feature needs to reference the two functions, move
them back to fs/mount.h.

Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>