rename do_filp_open() to do_file_open()

rename do_filp_open() to do_file_open()

"filp" thing never made sense; seeing that there are exactly 4 callers
in the entire tree (and it's neither exported nor even declared in
linux/*/*.h), there's no point keeping that ugliness.

FWIW, the 'filp' thing did originate in OSD&I; for some reason Tanenbaum
decided to call the object representing an opened file 'struct filp',
the last letter standing for 'position'.  In all Unices, Linux included,
the corresponding object had always been 'struct file'...

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