fs: factor out vfs_parse_monolithic_sep() helper

[ Upstream commit e001d144 ]

Factor out vfs_parse_monolithic_sep() from generic_parse_monolithic(),
so filesystems could use it with a custom option separator callback.

Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Stable-dep-of: c34706ac ("ovl: fix regression in parsing of mount options with escaped comma")
Signed-off-by: Sasha Levin <sashal@kernel.org>