hfsplus: Report case sensitivity in fileattr_get

hfsplus: Report case sensitivity in fileattr_get

Add case sensitivity reporting to the existing hfsplus_fileattr_get()
function via the FS_XFLAG_CASEFOLD flag. HFS+ always preserves case
at rest.

Case sensitivity depends on how the volume was formatted: HFSX
volumes may be either case-sensitive or case-insensitive, indicated
by the HFSPLUS_SB_CASEFOLD superblock flag.

FS_XFLAG_CASEFOLD is read-only: FS_XFLAG_RDONLY_MASK ensures
FS_IOC_FSSETXATTR strips it. The legacy FS_IOC_SETFLAGS path in
hfsplus_fileattr_set() also allows FS_CASEFOLD_FL through its
allowlist on case-insensitive volumes so that a chattr
read-modify-write cycle does not fail with EOPNOTSUPP.

Reviewed-by: Viacheslav Dubeyko <slava@dubeyko.com>
Reviewed-by: Roland Mainz <roland.mainz@nrubsig.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://patch.msgid.link/20260507-case-sensitivity-v14-7-e62cc8200435@oracle.com
Signed-off-by: Christian Brauner <brauner@kernel.org>