switch a bunch of places to mnt_want_write_file()

it's both faster (in case when file has been opened for write) and cleaner.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Conflicts:
	fs/ext4/ioctl.c
	fs/nfsd/nfs4recover.c
	fs/xfs/xfs_ioctl.c

Change-Id: Ibb2cc434d8d320d7c1790add3108330f6d5b2fc5