btrfs: open code fc_mount() to avoid releasing s_umount rw_sempahore

[CURRENT BEHAVIOR]
Currently inside btrfs_get_tree_subvol(), we call fc_mount() to grab a
tree, then re-lock s_umount inside btrfs_reconfigure_for_mount() to
avoid race with remount.

However fc_mount() itself is just doing two things:

1. Call vfs_get_tree()
2. Release s_umount then call vfs_create_mount()

[ENHANCEMENT]
Instead of calling fc_mount(), we can open-code it with vfs_get_tree()
first.
This provides a benefit that, since we have the full control of
s_umount, we do not need to re-lock that rw_sempahore when calling
btrfs_reconfigure_for_mount(), meaning less race between RO/RW remount.

Signed-off-by: Al Viro <[email protected]>
Reviewed-by: Qu Wenruo <[email protected]>
[ Rework the subject and commit message, refactor the error handling ]
Signed-off-by: Qu Wenruo <[email protected]>
Tested-by: Qu Wenruo <[email protected]>
Signed-off-by: David Sterba <[email protected]>

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