gfs2: gfs2_create_inode error handling fix

[ Upstream commit af4044fd0b77e915736527dd83011e46e6415f01 ]

When gfs2_create_inode() finds a directory, make sure to return -EISDIR.

Fixes: 571a4b57 ("GFS2: bugger off early if O_CREAT open finds a directory")
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>