vfs: clean up SETFLAGS and FSSETXATTR option processing

The FS_IOC_SETFLAGS and FS_IOC_FSSETXATTR ioctls were promoted from ext4
and XFS, respectively, into the VFS.  However, we didn't promote any of
the parameter checking code from those filesystems, which lead to a mess
where each filesystem open-codes whatever parameter checks they want and
the behavior across filesystems is no longer consistent.

Therefore, create some generic checking functions in the VFS and remove
all the open-coded pieces in each filesystem.  This preserves the
current behavior where a filesystem can choose to ignore fields it
doesn't understand.