BACKPORT: FROMGIT: f2fs: add reserved nodes for privileged users

This patch allows privileged users to reserve nodes via the
'reserve_node' mount option, which is similar to the existing
'reserve_root' option.

"-o reserve_node=<N>" means <N> nodes are reserved for privileged
users only.

Signed-off-by: Chunhai Guo <guochunhai@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit a3b0697610ba635c590f520f19aa3b2db46ca8bc https://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git

 dev)

Bug: 440276987
Bug: 446119465
Change-Id: I95914c2d196e4e3ddd1943c6c76d0b11acc8e93d
[panchuang: Had to adjust F2FS_MOUNT_RESERVE_NODE macro due to
bit allocation differences in 5.10]
[panchuang: Had to adapted to older API: used parse_options
instead of fs_context for mount option processing in 5.10]
Signed-off-by: panchuang <panchuang@vivo.corp-partner.google.com>