Utilize vfs_sanitizeopts() in vfs_mergeopts() to merge options. Because vfs_sanitizeopts() can handle "ro" and "rw" options properly, there is no more need to add "noro" in vfs_donmount() to cancel "ro".
Utilize vfs_sanitizeopts() in vfs_mergeopts() to merge options. Because
vfs_sanitizeopts() can handle "ro" and "rw" options properly, there is
no more need to add "noro" in vfs_donmount() to cancel "ro".

This also fixes a problem of canceling options beginning with "no".
For example, "noatime" didn't cancel "nonoatime". Thus it was possible
that both "noatime" and "nonoatime" were active at the same time.

Reviewed by:	bde

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